The dielectric constant of FR-4 epoxy material is derived by the 
ratio of glass to resin.

Dielectric Constant of E-Glass:	6.2

Dielectric Constant of Resin:	3.0
			
			Total	9.2

Assume standard FR-4 to be 50% glass and 50% resin, then divide by 
two.  The result is a dielectric constant of 4.6.  Though I have 
always used 4.5, for microstrip and embedded microstrip calculations, 
you can use 4.6 if you wish.  However, when calculating stripline, 
the traces are resin rich on the edges and sandwiched between two 
planes.  This changes the dielectric constant for stripline to 3.5.  
These numbers have been verified by thousands of actual designs 
requiring impedance control.

The thinner glass styles contain a higher percentage of resins and 
the thicker glass styles contain less resins.  Therefore, the thinner 
glass styles have a lower dielectric constant and the thicker glass 
styles have a higher dielectric constant.

To manufacture a controlled impedance board, your vendor must average 
the various dielectric constants of the various glass styles to 
obtain the correct dielectric constant to use.  This is why you may 
find the dielectric thickness actually used to construct the board, a 
little different than your stack-up used to obtain your original 
calculations. 

You will find that the final calculations come very close to the TDR 
readings on the actual manufactured board.

Hope this helps.

Norm Einarson